1. Field of the Invention
The present invention relates to an active guide wire for a catheter used in medical diagnosis or treatment as carried into a blood vessel and, in particular, to an improved guide wire that can be used for guiding the catheter to locate it at a desired site in narrow blood vessels of a complex vascular system such as intracerebral. The invention further relates to a method of making such an active guide wire.
2. Description of the Prior Art
The use of a guide wire to guide a catheter inserted in a blood vessel towards and through an affected or a potentially affected part has been known. In such use, the catheter into which the guide wire is inserted is allowed to orient in a desired direction by causing the leading end of the guide wire to bend to orient in that direction.
FIG. 11 is a view that diagrammatically illustrates how the catheter is advanced in blood vessels with the direction of its orientation controlled. While it is ideal to provide the catheter 111 itself with a bending drive mechanism at its leading end as shown, the catheter 111 can work equivalently to achieve an equivalent effect if a guide wire 112 is provided with a bending drive mechanism at its leading end is loaded in the catheter 111 as shown in FIG. 12. Even if the bending drive mechanism at the leading end of the guide wire is a bending mechanism capable of bending the leading end only in one direction, the bending action combined with a twisting operation 113 by the operator at the proximal side of the guide wire as shown allows its distal or leading end to orient in any direction as desired.
In order to guide the catheter at will along a complicate and fine duct line such as in the cerebrovascular system, the guide wire is desirable to be 0.5 mm or less in diameter and is required to possess an active drive function that is capable of bending the leading end and restoring it to be straightened as occasions demand. Furthermore, inasmuch as the vascular wall of aneurysm and those of a lesion such as stenosis is often extremely fragile, so that the region of the lesion or those vascular walls may not be injured by the guide wire, the wire guide must be made extremely low in its lateral and longitudinal stiffness.
In the medical site where it is necessary to guide the catheter at will along a fine and fragile blood vessel in a complicate vascular system such as pathologically changed cerebral blood vessels, an active guide wire small in diameter and low in stiffness if realized can be combined with a variety of existing catheters and is thereby expected to achieve great effects.
Conventional active guide wires with an active drive mechanism include, for example, one with a structure proposed by the present inventors as disclosed in our earlier Japanese Patent Application No. H10-355170 (JP 2000-233027 A).
The active guide wire disclosed there comprises as shown in FIG. 13, a bias coil spring 131 of radius r, and an actuator 132 disposed as fastened to the bias coil spring 131. The actuator 132 is made of a wire of a shape memory alloy (SMA) coiled and shape memorized so that it expands and contracts with a large stroke longitudinally of the coil; when heated by having an electric current passed therethrough the actuator 132 contracts or shrinks longitudinally thereof.
With this makeup, heating the actuator 132 by passing an electric current therethrough causes the actuator 132 to shrink, thereby producing a shrinking force F and thus causes the leading end of the active guide wire to bend with bending moment M=F×r. And removing the electric current permits the leading end of the active guide wire under the restoring force by the bias coil spring 131 to restore its straightness.
It has been found, however, that using this makeup to form an active guide wire that is small in diameter (=2r) makes it hard to enough bend the leading end of the active wire guide for the reason that reducing the radius r of the bias coil spring 131 reduces the bending moment M.
Attempting to hold the bending moment M needed to enough bend the leading end of the active guide wire calls for increasing the shrinking force F of the actuator 132. Attempting to increase the shrinking force of the actuator 132 in turn requires making thicker the wire diameter of the coiled wire made of a shape memory alloy. Making the wire diameter thicker of the coiled wire 132, however, makes it larger in both its lateral and longitudinal stiffness.
Also, this makeup in which the actuator 132 is allowed to shrink requires the bias coil spring 131 to be axially of not less than a certain stiffness such that the shrinkage of the actuator 132 may not shrink or buckle the active guide wire as a whole.
Thus, this makeup is found to be hard to satisfy simultaneously the reduced diameter which is required for the active guide wire used to carry the catheter in a narrow blood vessel of a complicate vascular system such as intracerebral and to guide it to reach a desired site for diagnosis or treatment, and the adequate stiffness which is required not to injure a pathologically changed vascular wall.
The active guide wire may utilize another makeup proposed by the present inventors and disclosed in earlier Japanese Patent Application No. 2000-092209. The makeup of the active guide wire there disclosed differs from the makeup disclosed in JP 2000-233027 A in that the actuator made of a shape memory alloy is in the form of an elongate flat sheet shaped to run zigzag along the axis thereof. The actuator is shape memorized so that when heated with an electric current passed therethrough it shrinks along the axis thereof with a large stroke.
With this makeup, as in that described in JP 2000-233027 A, reducing the guide wire in diameter must result in an increase in stiffness of the actuator to keep the enough bending moment M. Furthermore, the bias spring coil must again be axially of not less than a certain stiffness to avoid shrinking or buckling as the whole. Consequently, this makeup, too, makes it hard to bring into reality, not only the reduced diameter which is required for the active guide wire used to carry the catheter in a narrow blood vessel of a complicate vascular system such as intracerebral and to guide it to reach a desired site for diagnosis or treatment, but also the adequate stiffness which is required not to injure pathologically changed vascular walls.
Thus, while active guide wires of these prior designs are each highly useful when used to carry a catheter in an aorta, a renal artery or the like blood vessel that is relatively thick itself and in wall thickness, said prior guide wires used in a cerebral or a pathologically changed blood vessel may potentially injure the blood vessel.